1. Field of the Invention
The present invention relates to a coated cemented carbide cutting tool member (hereinafter referred to as a xe2x80x9ccoated carbide memberxe2x80x9d) that has superior ability to avoid breakage and chipping around its cutting edge even when it is applied to extremely tough cutting operations for metal workpieces like those of steel and cast iron, such as high-speed cutting operations with thick depth-of-cut, high-speed cutting operations with high feed rate, interrupted cutting operations at high-speed and so on, all of the operations producing severe mechanical and thermal impacts at the cutting edge.
2. Description of the Related Art
It is well known that coated carbide members are preferably composed of a tungsten carbide-based cemented carbide substrate and a hard coating layer which comprises an inner layer having an average thickness of 0.5 to 20 xcexcm and preferably composed of a titanium compound layer including at least one layer of titanium carbide (hereinafter referred to as xe2x80x9cTiCxe2x80x9d), titanium nitride (TiN), titanium carbonitride (TiCN), titanium carboxide (TiCO) and titanium carbonitroxide (TiCNO), and an outer layer having an average thickness of 0.3 to 15 xcexcm and composed of aluminum oxide (Al2O3) layer which has several crystal polymorphs such as xcex1, xcexa, and xcex3. The hard coating layer could be formed preferably by means of chemical vapor deposition and/or physical vapor deposition. The coated carbide member is widely used in various fields of cutting operations, for example, continuous and interrupted cutting operations on metal workpieces such as those of steel and cast iron.
It is also well known that titanium compound layer has a granular crystal morphology and is used for many applications. Among them, TiC, TiCN and TiN layers have been widely used as highly abrasion resistant materials in many applications, especially in wear resistant layers of cutting tools. Furthermore, TiN layers have been widely used as surface decorative coatings because it has a beautiful external appearance similar to that of gold. For many coated carbide members, the outermost layers are made of TiN, and this facilitates distinguishing by machining operators of new cutting edges from the cutting edges which are already worn, even in dim environments.
A TiCN layer that has a longitudinal crystal morphology, produced by chemical vapor deposition in a moderate temperature range such as 700 to 950xc2x0 C. using a reaction gas mixture which includes organic cyanide compounds such as acetonitrile (CH3CN), has been well known as a highly tough and wear resistant coating layer, which was disclosed in Japanese Unexamined Patent Publications No. 6-8010 and No. 7-328808.
It is well known that a typical method for covering the substrate""s surface with Al2O3 layer is a chemical vapor deposition (CVD) process using a gas mixture of AlCl3, CO2 and H2 at around 1000xc2x0 C., and that the typical conditions utilized in CVD-Al2O3 processes could mainly produce three different Al2O3 polymorphs, namely, the most thermodynamically stable xcex1-Al2O3, meta-stable xcexa-Al2O3 and xcex3-Al2O3. It is also well known that the specific polymorph of produced the Al2O3 layer is controlled by several operative factors, such as the surface composition of the underlying layer, the deposition condition of Al2O3 nucleation status and the temperature of the Al2O3 growth status.
In recent years, there has been an increasing demand for laborsaving, less time consuming, cutting operations. Accordingly, the conditions of these cutting operations have entered difficult ranges, such as high-speed cutting operations with thick depth-of-cut, high-speed cutting operations with high feed rate, and interrupted cutting operations at high-speed. For coated carbide members, there are few problems when they are applied to continuous or interrupted cutting operations on steel or cast iron under common cutting conditions.
If a conventional coated cemented carbide cutting tool is used under high speed cutting conditions, thermal plasticity tends to occur easily at the cutting edge due to lack of heat resistance of the outer layer composing the hard coating layer because of the heat generated during the cutting. In particular, the outer layer comprising the hard coating layer and the inner, layer both of which have relatively good thermal conductivity, and in addition, the thermal conductivity of Al2O3 forming the outer layer is 6 W/mK, and the thermal conductivity of TiN is 14 W/mK; thus, the high heat generated between the workpiece and the hard coating layer influences the carbide base, and the thermal plasticity transformation inevitably occurs on the cutting edge. Therefore, abrasion becomes partial due to the thermal plasticity; thus, the abrasion of the cutting edge advances noticeably, and the tool life of such cutting tool is relatively short.
Also, even though the Al2O3 layer as the outer layer composing the hard coating layer has superior hear resistance, if a conventional coated cemented carbide cutting tool is used under high speed intermittent cutting conditions with large mechanical and thermal impacts, because the AL2O3 as the outer layer composing the hard coating layer has more contact with the workpiece than the Ti chemical compounds as an inner layer during the cutting operation, the AL2O3 layer directly receives large mechanical and thermal impacts; thus, the tool life of such a cutting tool is short and chipping occurs easily on the cutting edge because of inferior toughness of the conventional coated cemented carbide cutting tool; thus, the tool life of such a cutting tool is short.
Therefore, there are severe problems of failure in relatively short times when they are used in tough cutting operations of these materials, and these are accompanied by severe thermal and mechanical impacts, because the Al2O3 layer, whose mechanical toughness is not sufficient in spite of its superior properties for thermal stability and thermal barrier effects, suffers detrimental thermal and mechanical impacts owing to its preferential contact as an outer layer with work materials, and this phenomenon induces the breakage or chipping around the cutting edge.
Accordingly, an object of this invention is to provide a coated carbide member that does not breake or chip around its cutting edge for a long period of time even when it is used in extremely tough cutting operations for metal workpieces such as those of steel and cast iron.
The object of the present invention has been achieved by the discovery of a coated carbide member whose cemented carbide substrate is coated with a hard coating layer having a total thickness of between 0.5 to 20 xcexcm and preferably comprising an alternated multilayer structure of the first thin layer and the second thin layer whose individual thickness is between 0.01 to 0.3 xcexcm, and the first thin layer is made of titanium compounds such as TiC, TiCN, and TiN, and the second thin layer is made of hard oxide materials such as Al2O3 and hafnium oxide (HfO2).
This coated carbide member gives good wear resistance and long tool lifetime even when it is used in extremely tough cutting operations for metal workpieces like those of steel and cast iron.